Joint and joint assembly

A joint assembly comprising:

• a tube including a resin;
• and
• a joint connected to the tube, the joint including
• a joint body having a tube coupling part and a main body thread part, the tube coupling part constituting one axial end side of the joint body and having at least a portion of a periphery covered by the tube, the main body thread part having an outer diameter greater than an outer diameter of the tube coupling part, and an axially extending through hole being formed in the joint body,
• a nut having a nut thread part and a constricted diameter part, the nut thread part threadedly engageable with the main body thread part in a state in which the tube coupling part of the joint body is inserted from an axial direction, the constricted diameter part being disposed on an insertion and travel side and having an inner diameter smaller than an inner diameter of the nut thread part, and the tube coupling part being inserted and traveling toward the insertion and travel side, and
• a ring arranged in a space radially inward of the nut thread part of the nut, the ring abutting an abutted part that is a portion of the constricted diameter part of the nut on a joint body side, and the ring having a Young's modulus greater than the tube,
• the ring having a curved surface part at least at a radially inward portion of the ring axially on the joint body side, the curved surface part bulging radially inward of the ring and axially toward the joint body side, and
• a maximum thickness of the ring in a radial direction being defined as 2r, an axial length of the curved surface part of the ring being defined as h, and α=h/r being 0.5 or more and 1.2 or less.